Laser diodes which exhibit a relatively linear relationship between input current and optical output have recently come into extensive use in connection with the transmission of broadband video signals over fiber optic transmission paths. In such systems, the input current signal feeding a laser diode may be amplitude modulated with a broadband radio frequency signal comprising a plurality of radio frequency channels. Each of these r.f. channels is in turn modulated with a baseband video signal. Laser diodes having characteristics which are sufficiently linear to serve as devices for converting such amplitude modulated input signals into corresponding amplitude modulated optical signals are quite expensive. It is a characteristic of suitable laser diodes which are presently available that supplying these devices with input signals having magnitudes outside of a specified range may cause damage to them. At least two factors may influence the magnitude of the input signal to the laser diode to exceed acceptable limits. Firstly, if the peak-to-peak magnitude of the r.f. signal becomes too great, the negative excursion of the signal may be outside the safe operating range of the laser. Even if the r.f. input signal is maintained at an acceptable level, the safe operating range of the laser may still be exceeded if the d.c. bias current to the laser diode is lost. Such a loss of d.c. bias current may occur, for example, as a result of a defect in the power supply which produces the d.c. bias current.
A number of circuits have been proposed for regulating the peak-to-peak amplitude of input signals to a laser diode. Typically, however, these circuits are adapted for maintaining the input signal range within the linear operating range of the laser diode characteristic. These circuits are not specifically adapted, however, to protect the laser diode in the event that its input signals exceed an acceptable magnitude. The circuits also do not protect the laser in the even that the magnitude of the input signals exceed a safe value because of, for example, a loss or reduction of input bias current.